High temperature organic electrolyte cells

ABSTRACT

An anode coating for anhydrous electrochemical generating cells is disclosed wherein the anode active material is coated with the insoluble reaction product of the anode metal with an inhibiting reactant such as CO 2 , SO 2 , O 2 , NH 3 , and N 2  saturated with respect to water vapor H 2  O. This coating is insoluble in the cell under open circuit conditions and is stable with regard to said electrolyte but is soluble in said electrolyte, when the cell is discharged with a negligible over voltage. The coated anodes and the method for making same are specifically described. Cells with anodes treated according to this invention have been stored at 55° C, for extended periods with negligible self discharge.

This is a continuation of application Ser. No. 254,998, filed 5/19/72,now abandoned.

This invention relates to high energy density cells and moreparticularly to organic electrolyte cells having active metal anodes andcells which are capable for storage at elevated temperatures forextended periods.

In the development of electrochemical cells, research has been directedto improvement of several observable characteristics which determinepractical cell value. These characteristics which include cell energydensity, utilization or discharge efficiency of active cathode material,and cell shelf life, are dependent upon a multiplicity of factors eachof which has been the subject of extensive past study.

Cell energy density, the product of cell capacity and operative voltageper weight of cell active materials, is determined primarily by theelectric potentials in the cell, electrode materials and theircapacities per unit weight. In the transition from aqueous electrolytesto organic electrolytes, a significant improvement in cell energydensity has occurred. In particular, metals of the electrode-positiveseries i.e., those metals more electro-positive than hydrogen and havingsuch high negative electric potential as to decompose water and therebybeing incompatible with aqueous electrolytes, have been found suitablefor use as anodes when an organic liquid medium is used as theelectrolyte. Cell output voltage, equal to the difference in electrictension of the cell cathode and anode has been increased accordingly.Cell current capability is largly dependent upon ion transfer betweenthe anode and cathode and upon the cathode conductivity. With respect toion transfer, this factor is largly dependent upon the reducing power ofthe anode material, the ionic conductivity of the electrolyte and theoxidizing power of the active cathode materials. Recent research hasdeveloped cells employing anode materials and organic electrolytes whichexhibit excellent ion transfer characteristics. Patent applications havebeen filed for cells utilizing the anode materials and certain specificorganic electrolytes. United States patent applications U.S.N. 829,849now abandoned, and U.S.N. 853,312, now U.S. Pat. No. 3,808,052, andothers, are directed to such organic electrolyte cells.

The inhibition of the self discharge at high temperatures according tothis invention is accomplished by treating the active metal anodes ofthe cells in a solution of the electrolyte through which is bubbled, ingaseous form, certain reactants which form a coating layer upon theanode which has been observed to be insoluble in the electrolyte duringopen circuit storage of cells containing so-treated anodes. Upon beingsubjected to the slight over voltage resulting from the closing of thecircuit, the coating layer is removed and has not been found tointerfere materially with either the capacity or initiation time of thecell. Among the most effective electrodes for which such a treatment isuseful are those active elements and anodic materials lying abovehydrogen in the electromotive series and are those which will displacehydrogen from aqueous solutions. Most useful among such materials arethe light metals which comprise magnesium, lithium, sodium, potassium,and calcium. All the above are particularly useful in high energydensity electrochemical organic electrolyte cells. Preferred and mostuseful in the present invention are the lithium anodes.

The prior art organic electrolyte cells have shown good stability andlong shelf-life on storage at ambient temperatures. However, onprolonged storage at elevated temperatures, such cells have shown signsof deterioration. While each component separately, i.e. the cathode withelectrolyte and the anode with electrolyte was shown to be stable ateven high temperatures, when they were combined and assembled into acell, such cells were found to be unstable during high temperaturestorage. It has been postulated that the self discharge of the cathodeswere enhanced by interactions of the cathode with the anode via theelectrolyte at elevated temperatures.

SUMMARY OF THE INVENTION

It is an object of this invention to provide high energy density primaryorganic electrolyte cells.

It is a further object to provide such high energy density cells havingprolonged shelf-life on elevated temperature storage.

The above and other objects of this invention are achieved by theutilization of a novel anode treatment, that forms a heat stable,insoluble film on the anodes, the anodes resulting from such treatmentand the cells utilizing such anodes.

Among the reactants for the treatment most suitable for forming anelectrolyte open circuit insoluble film upon the anodic materials areinhibitor gases including; CO₂, SO₂, O₂, NH₃, and, N₂ saturated withrespect to water vapor.

The inhibition of self discharge according to this invention ispreferably accomplished by pre-treating the anode material in theelectrolyte to be used in the final cell through which is bubbled thereactant gas. Characteristically, most of the preferred reactants orinhibitor compounds are only slightly soluble in the preferredelectrolyte materials. Further when the anodes or treated with thereactant in the electrolyte it has been noted that a film is formed uponthe surface of the anode metal and that after the initial film orcoating of the reactant upon the anode material has been formed thereappears to be no sub-surface penetration of the film material. This isprobably due to the fact that the film material is only very slightlysoluble in the electrolyte in the absence of any electromotivepotential. However, the coating is rapidly dissolved in the electrolyteproper under the application of a slight overvoltage. Since the coatinglayer is of minimal thickness, in the order of probably only severalmolecules, it does not, to any appreciable extent, interfere with theonset of the discharge reaction or with the total capacity of the cell.

From the above it is clear that the lithium anode or any of the otheractive metal anodes are not passivated, since they can be dischargedwith a negligible overvoltage. The open circuit coupling reaction viathe electrolyte between the cathode and anode is prevented. This theoryappears consistent with the observed facts.

The invention will be more fully described in connection with thefollowing examples.

EXAMPLE 1

The inhibition of self discharge was accomplished by pre-treating agroup of lithium anodes in an electrolyte through which CO₂ was bubbled.The electrolyte consisting of 1 molar LiClO₄ l in tetrahydrofuran. Theseanodes were then mounted in standard test cells. The cells were sealedand stored in thermostatic controlled ageing cabinents for 30 days at55° C. The cells were prepared with the various cathode couples Thoseincluded are the depolarizers which are substantially insoluble in theorganic electrolyte such as, molybdic oxide, vanadium oxide, variousmetal chromates, silver permanganates, ammonium molybdate, silverperiodate, and in all cases the anode-cathode couples were protected andthe cells showed negligible self discharge.

EXAMPLE 2

A same procedure described in Example 1, was used to treat anodes oflithium, but the solvent was changed from tetrahydrofuran to propylenecarbonate in the first instance and acetonitrile in the second. Anodestreated in these solvents in which was dissolved 1 molar lithiumperchlorate electrolyte and subjected to the carbon dioxide bubblingaction similarly provided cells stable at high temperatures.

EXAMPLE 3

A battery containing lithium anodes prepared according to example 1, wasassembled from lithium-molybdic oxide electrodes in an electrolytecomprising 1 molar lithium perchlorate dissolved in tetrahydrofuranpropylene carbonate (1:1) as the electrolyte. The battery was stored for35 days in the high temperature storage cabinet at 55° C. Uponsubsequent discharge, it was found to have undergone only negligibleself discharge. The nominal capacity of the battery was exceeded and thecell performed normally at low and high potential discharge rates.

EXAMPLE 4

Lithium anodes were treated in a manner similar to that described inExample 1, but instead of carbon dioxide, SO₂ was bubbled through thesolvent. Cells prepared under these conditions showed good hightemperature stability upon high temperatures.

EXAMPLE 5

Lithium anodes were treated in a manner similar to that described inExample 1, but instead of carbon dioxide ammonia (NH₃), was bubbledthrough the solvent. The ammonia gas contained slight traces of oxygen.The cells after accelerated storage test as described in Example 1showed comparable qualities.

EXAPLE 6

Lithium anodes were treated in a manner similar to that described inExample 1, but instead of CO₂ traces of moisture were introduced bybubbling nitrogen, saturated with respect to water vapor, through theelectrolyte mixture. In Examples 4, 5, and 6 the cells assembled fromso-treated anodes all displayed good capacities after high temperaturestorage.

What is claimed is:
 1. A high energy density electrochemical cell havingnegligible self-discharge characteristics at high temperatures,consisting essentially of (a) a solid cathodic electrode containingactive depolarizer material selected from the group consisting of metaloxides, chromates, vanadates, permanganates, molybdates, periodates, andhalides, (b) a sulfur dioxide free organic electrolyte comprising anorganic solvent having dissolved therein an electrolyte salt, and (c) ananode on which is formed a heat stable coating which is insoluble in theorganic electrolyte under open circuit conditions but which isirreversibly dissipated upon cell discharge, said insoluble film havingbeen formed as a reaction product only prior to initial discharge ofsaid cell with an inhibitor gas selected from the group consisting ofCO₂, SO₂, O₂, NH₃, and N₂ saturated with respect to water vapor, wherebyself discharge of said cell is inhibited until said initial celldischarge.
 2. A cell as in claim 1 wherein said anode is an active metalselected from the group consisting of Li, Na, K, Mg, and Ca.
 3. A cellas in claim 2 wherein said anode is Li.
 4. A cell as in claim 2 whereinsaid organic solvent is selected from the group consisting oftetrahydrofuran, acetonitrile, propylene carbonate, diemthyl sulfoxide,dimethyl formamide, gamma-butyrolactone, dimethyl carbonate, N:Nnitrosodimethylamine, dimethyl sulfite, N:N-dimethyl formamide andmixtures thereof, and said electrolyte salt is one selected from thegroup consisting of the active metal chlorides, bromides, iodides,perchlorates, hexafluorophosphates, tetrafluoroborates,hexafluoroarsenates, and tetrachloroaluminates.